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Allylic Carbon

The Modern Periodic table in which all known elements are arranged in increasing order of atomic number. There are 18 vertical columns called as groups and 7 horizontal rows called as periods. 

The bottom part of periodic table contains two series of 14 elements which are called as f-block element. The left side of Modern Periodic table mainly contains metals whereas right side is non-metallic region. Some elements show intermediate properties of metals and non-metals. These elements are called as metalloids and are located in between metals and non-metals as a zig “ zag line. Metals can be identified as elements which are highly reactive and electropositive in nature. 

Unlike metals, non-metals are electronegative in nature. Carbon is one of the most common non-metals. It is basic of all organic compounds. Organic chemistry is the branch of chemistry that deals with different organic compounds.

Organic compounds are composed of different element with parent carbon chain. Hydrocarbons are most common organic compounds. They are composed of carbon and hydrogen mainly. 

Carbon exhibits tetravalency so it can form four covalent bonds with same or different elements. Due to tetravalency, carbon shows catenation and can form different organic compound. Catenation is the property of carbon or other elements to form covalent bond with same element.

On the basis of number of carbon atoms bonded with a carbon atom, it can classify as primary, secondary and tertiary carbon atom. A carbon atom which is bonded with one another carbon atom is called as primary carbon atom. For example in ethane molecule $(CH_{3}-CH_{3})$ both carbon atoms are bonded with one other carbon atom so both carbon atoms are primary carbon atom here. Secondary carbon atom is bonded with other two carbon atoms whereas tertiary carbon atom is bonded with other three carbon atoms. 

Primary, Secondary and Tertiary Carbons

 

Allylic Carbon Meaning

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The double bonded carbon atoms can be classified as vinylic and allylic carbon atoms. The general formula for vinyl group is $RCH = CH_{2}$ in which both carbon atoms are bonded with double bond and R is attached at vinylic position. Since both carbon atoms form double covalent bond so both are $sp^{2}$ hybridized. The allylic position is also like vinylic position. The allylic carbon is bonded to a carbon atom which is doubly bonded to another carbon atom. General formula for allyl is $RCH*2CH=CH_{2}$ in which the asterisk carbon atom is allylic carbon atom. Unlike vinyl group, the allylic carbon atom is $sp^{3}$ hybridized as it bonded with $CH = CH_{2}$ through a single covalent bond. It imparts unique chemical properties to allylic group and presence of this group in different compounds form allylic compounds which are used for the preparation of different natural products like natural rubber, terpenes etc.

Allylic Carbon Atoms

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The allyic carbon atom is the $sp^{3}$ hybridized carbon atom in allylic group $RCH_{2}-CH$=$CH_{2}$ that is bonded with $–CH=CH_{2}$ group.  For example in propene, the highlighted carbon atom is allylic carbon atom $(CH_{3}-CH$=$CH_{2})$. Similarly in cyclohexene, the carbon atoms next to double bond are allylic carbon atoms. 

Allylic Carbon Atoms

Allylic Carbocation

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Carbocations are ionic species which carry positive charge on carbon atom of molecule. They usually form as an intermediate during various chemical reactions. The stability of carbocations determines by steric hindrance and +I effect of alkyl groups attached to C+ of carbocation. As the +I effect increase of positively charged carbon atom of carbocation, it reduces the positive charge on carbocation. So we can say that as the number of alkyl groups increases on C+, the stability of carbocation also increases.  Therefore the stability order of carbocation can be written as;

Tertiary carbocation >   Secondary carbocation > Primary carbocation

In the allylic group, if the allylic carbon atom carries positive charge, it forms allylic carbocation. The allylic carbocation is stable due to delocalization of electrons on carbon atoms. 

Allylic Carbocation

Similarly in case of carbocation of cyclohexene, the formal charge on allylic carbon is +1 and it stabilizes by resonance with pi-bond. 

Allylic Carbocation
If the allylic carbon atom is attached with one carbon atom and carries +1 charge, it is called as primary allylic carbocation. Here formal charge of +1 is on primary carbon atom so it is named as primary allylic carbocation. 

Allylic Carbocation
 
In case of secondary allylic carbocation, the +1 formal charge is distributed on secondary carbon atom as in cyclohexene cation. 

Allylic Carbocation
 
A tertiary allylic carbocation has +1 charge on a tertiary carbon atom of cation. 

Allylic Carbocation

Allylic Carbon Radical

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A free radical can be an atom, molecule, or ion with an unpaired valence electron. Free radicals are formed by hemolytic cleavage of covalent bond that usually occurs in the presence of light. Like $Cl_{2}$ molecule forms two Cl• radicals. 

Allylic Carbon Radical
 
Similarly if the unpaired electron is on carbon atom, it is called as carbon radical. On the basis of bonding, carbon radical can be classified as primary, secondary and tertiary radicals. The order of stability of free radicals is same as for carbocations as it is also effected by +I effect of alkyl groups attached on C•. 

Allylic Carbon Radical
 
In an allylic radical the unpaired electron remains in resonance with pi-bond that makes it stable. The resonance is allylic radical is shown below. The pi-bond involves in resonance with the unpaired electron of radical. An electron from the pi-bond forms a new bond with the carbon radical. It leaves an electron to the carbon at the allylic position. 

Allylic Carbon Radical
 
Because of this resonance, allylic radicals are more stable compare to other radicals.

Oxidation of Allylic Carbon

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The bond dissociation energy of C-H bond in propane is 410 kJ /mol whereas in case of propene, the C-H bond of allylic position requires 368 kJ /mol of energy. It indicates that allylic bond is weaker and more reactive compare to other C-H bond. 

Oxidation of Allylic Carbon
 
The stability and reactivity of allylic radical play important role in the preparation of allylic halides. The oxidation at allylic position occurs in the presence of Selenium dioxide as catalyst. Reaction involves the formation of allylic seleninic acid as an intermediate which further rearrange and oxidize to form carbonyl compounds. 

Oxidation of Allylic Carbon
 
Selenium oxide is mainly used for allylic oxidation of alkenes to form allylic alcohols without migration of the double bond of the parent molecule.
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